One of ultrafast communication transistors operable in, for example, a millimeter-wave band (approximately 30 GHz to approximately 300 GHz) or a submillimeter-wave band (approximately 300 GHz to approximately 3 THz) is an InP-based high electron mobility transistor (HEMT) which uses a Group III-V compound semiconductor.
The InP-based HEMT has a basic structure in which InGaAs is used for an electron traveling layer (channel layer) and InAlAs is used for an electron supply layer (barrier layer). Here, an In0.53Ga0.47As channel layer lattice-matched to an InP substrate, a pseudo-lattice-matched group channel layer in which an InAs composition in an InGaAs channel layer is increased to approximately 0.7 to approximately 0.8 in order to increase high speed performance, a composite channel layer including an InAs ultrathin layer in an InGaAs layer, and the like are used as the channel layer.
The current world's fastest field effect transistor is an In0.7Ga0.3As/InAs/In0.3Ga0.3As composite channel HEMT, and a cut-off frequency fT thereof is approximately 710 GHz.
Japanese Laid-open Patent Publication No. 2007-81103 and Japanese Laid-open Patent Publication No. 2-246342 are examples of related art.
E.-Y. Chang et al., “InAs Thin-Channel High-Electron-Mobility Transistors with Very High Current-Gain Cutoff Frequency for Emerging Submillimeter-Wave Applications”, Applied Physics Express 6, 034001 (2013) is also an example of related art.
Incidentally, a cut-off frequency fT which is one of the criteria for the high speed performance of HEMT is expressed by the following expression.
      f    r    =      1          2      ⁢              π        ⁡                  (                                                    L                g                            v                        +                          τ              ex                                )                    
Here, Lg denotes a gate length, v denotes electron velocity under a gate electrode, and τex denotes a parasitic delay time.
The high speed performance of an HEMT is mainly realized by a reduction in the size of the gate length Lg and an increase in the electron velocity v caused by a semiconductor material, having a small effective mass of electrons, which is used for a channel. The reduction in the size of the gate length and the increase in the electron velocity are equivalent to a reduction in an intrinsic delay time of the HEMT. At present, the size of the gate length Lg has been being reduced in order to realize the high speed performance of the HEMT.
When the size of the gate length Lg is reduced, a distance between a gate and a channel is also reduced from the viewpoint of scaling. For this reason, a barrier layer is thinned.
On the other hand, the barrier layer is δ-doped with Si as a source for supplying electrons to a channel layer. In this case, when the barrier layer is thinned, the amount of electrons supplied to the channel layer is decreased. For this reason, sheet resistance is reduced, and thus an Si δ-doping amount is increased, thereby increasing the concentration thereof.
In the above-mentioned InP-based HEMT, Si with which the δ-doping is performed widely diffuses to an InAlAs barrier layer and diffuses to an InGaAs channel layer in some cases in accordance with the reduction in the thickness of the barrier layer and the high concentration Si δ-doping.
In this manner, when Si diffuses into the channel layer in accordance with the reduction in the thickness of the barrier layer and the high concentration Si δ-doping, electron velocity and electron mobility in the channel layer are reduced due to the ionized impurity scattering of Si which diffuses into the channel layer. This phenomenon is a negative factor in realizing the high speed performance of the HEMI.